NEW YORK (AP) -- President Barack Obama said the Republican Party is threatening voting rights in America more than at any point since the passage of a historic 1965 law expanding rights at the ballot box to millions of black Americans and other minorities.

Obama's critique of Republicans on Friday came as he seeks to mobilize voters ahead of the November congressional elections, when Democratic control of the Senate is at stake, as is the president's already limited ability to push his agenda through Congress. Many in Obama's party fear state voting requirements and early balloting restrictions will curb turnout that is critical to Democratic hopes of prevailing.

"The stark, simple truth is this: The right to vote is threatened today in a way that it has not been since the Voting Rights Act became law nearly five decades ago," Obama told a crowd of about 1,600 people at civil rights activist Al Sharpton's National Action Network conference, held in a New York hotel ballroom.

It was the second day in a row that America's first black president has delivered a speech about race, an issue that has not often been at the forefront of his agenda. Obama has faced criticism from some African-Americans for doing too little to help minorities, but he has focused more acutely on inequality in his second term.

For the remainder of the year, no political issue stands out more prominently for Democrats than their ability to motivate voters to turn out at the polls in the November midterm elections. A Republican takeover of the Senate would crush Obama's already limited ability to push his agenda through Congress. The Republicans are virtually certain to keep their majority in the House of Representatives, but the fight for the Senate is expected to be tight.

Turnout by Democrats has been traditionally weak in elections when the White House is not at stake. That, coupled with efforts in some states to limit early voting and to enact voter identification requirements, has prompted Obama and his party to raise alarms and step up their get-out-the-vote efforts.

The president vowed that he would not let the attacks on voting rights go unchallenged, but he offered no new announcements of specific actions his administration planned to take.

Just last year, seven states passed voter restrictions, ranging from reductions in early voting periods to identification requirements, according to the Brennan Center for Justice at New York University School of Law. North Carolina alone adopted a photo ID requirement, eliminated registrations on Election Day and reduced the number of early voting days. Overall 34 states have passed laws requiring voters to show some form of identification at the polls.

The president pinned efforts to curb access to the ballot box directly on the Republicans, declaring that the effort "has not been led by both parties. It's been led by the Republican Party.'' Mocking the Republicans, he said, ``What kind of political platform is that? Why would you make that a part of your agenda, preventing people from voting?"

Republicans have long argued that identification requirements and other voting controls are reasonable measures designed to safeguard the balloting process, not to suppress voter turnout. Democrats say photo identification requirements especially affect minority or low-income voters who may not drive and thus wouldn't have an official government ID.

A spokeswoman for Republican Sen. John Cornyn of Texas, a state whose voting laws are being challenged by the Obama administration, said the Supreme Court has ruled that voter identification laws are constitutional.

"Protecting the integrity of the voting process is something that benefits everyone, partisan politics do not," the spokeswoman, Megan Mitchell, said.

The Obama administration has also challenged the North Carolina election law. That state's measures, which take effect in the 2016 election, came after the Supreme Court last June threw out the crucial section of the Voting Rights Act that required that all or parts of 15 states with a history of discrimination in voting, mainly in the South, get federal approval before changing their election laws.

Obama's speech to a crowd of about 1,600 in a New York hotel ballroom came a day after he marked the 50th anniversary of the Civil Rights Act, the landmark law that ended racial discrimination in public spaces.

The anniversary has brought renewed attention to the accomplishments of the civil rights movement. A CBS News poll released Wednesday found that more than 3 in 4 Americans say there has been progress in getting rid of racial discrimination. But those views split racially, with whites much more likely than African-Americans to think real progress has been achieved.